An alias filter is a soft-edged glass in front of the sensor of digital cameras. It is required to remove frequencies higher than half the sensor pixel density (Nyquist limit), before sampling (digitization). The alias filter consists of a light unsharp mask (Blur) and acts as an optical low-pass. For some years individual manufacturers omit the alias filter in their cameras and praise this as improvement of sharpness and resolution. But is this really?Low-pass filtering is a universal and basic requirement for all kinds of digital sampling to avoid artifacts, i.e. false content in the sampled signal. This also applies, for example, to digital audio recording. Audio is usually sampled at 44.1 or 48 kHz and filtered at approximately 20 ... 22 kHz to avoid audible artifacts.

In photography, scanning errors due to inadequate or lack of low-pass filtering becomes visible as moiré, i.e. streaks or smears in fine structures. In addition, the moiré of color cameras shimmers like a rainbow of colors (iridescence or color stripes) and is particularly striking (clearly visible in this test of the Leica M8 - scroll down the bottom of the page).

Aliasing filters are generally required for all types of digital image sensors and not just for sensors whose color scanning is performed by Beyer filters, as some manufacturers want us to believe. Even cameras with beam splitting to three sensors (one for each primary color), cameras with unusual color sampling (e.g. Fuji X Series), cameras with a pure black & white sensor (eg Leica M) and cameras with Foveon X3 sensor, where all three color channels are scaned at the same location (eg, Sigma DP1), all need an alias filter. Otherwise, even in the pictures of these cameras moiré is clearly visible when fine structures are captured.

To quote from Fuji's press text: "Without using an optical low-pass filter, moiré is eliminated while preserving high resolution". Sure, Fuji, that's very obvious:

At first sight the picture on the left may actually look higher resolved than the one on the right. In reality, however, the visible structures inside the colored artifacts are just sampling errors and not real image detail.

Colored iridescence is resulting from the superposition of random and therefore varying artifacts in the three color channels. It occurs on all cameras where the three color channels are recorded separately in space and is not limited to Beyer sensors. Example of color moiré from Fuji X cameras can be seen in the above picture to the left, at Luminous Landscape and also in my own further testings. Streaks occur on high-contrast edges, where the sub-pixels of the three primary colors are exposed to different brightnesses. Only cameras with Foveon sensors and pure black&white cameras are immune to color and iridescence artifacts (but not to moiré in general).Where do such flaws become visible in real life? Wherever fine structures at the limit of the sensor resolution are to be photographed. For example, fine-textured materials (fashion photography, close-ups of the structures of plant leaves), but also in roofs (shingles, tiles) or fences (meshs), when such objects are positioned small in the background of an image. Streaks can be seen on high-contrast edges, for example with dark skyscrapers against a bright sky. But moiré also occurs in very common motives, like a finely striped sweater of a lady sitting in the background.

To omit an alias filter in front of a digital image sensors is like building a sports car with no brakes. Of course, the car accelerates a little faster due to the lower weight and the cornering ability is also better due to the smaller unsprung weight - but ultimately it lacks an essential functional element.

For analog cameras, an alias filter is not required: ​​Film has no sharply defined limit of resolution. It loses contrast and resolution gradually with increasingly higher frequencies. You could say, the low-pass filter is already incorporated in the film itself.

Sometimes it is claimed that picture errors arising from cameras without aliasing filters can be corrected later by software when needed. That, however, is theoretically and practically impossible because sampling errors mix undetected into the real image details. Of course, you can try to identify fine image structures automatically by software and then remove the moiré (and hopefully also the color streaks) by applying a Blur filter. But the result is necessarily worse than if the recording had been made with a proper alias filter upfront. This is because all of the details in the moiré areas have to be blurred - and that includes also the real structures which can not be differentiated from the false artifacts.

Also important to know: By omitting the alias filter, the recorded image information (the recorded true sensor resolution) does not increase!

Even though images of cameras without aliasing filters may appear sharper and crisper: Images of cameras with a proper alias filter can easily be re-sharpened to achieve the same visual impression - without side effects. A good example is the Canon 5D Mark III, whose alias filter was designed precisely to the point. The filter of the two predecessors (5D and 5D mark II) were a bit weak, so sharpening was hardly possible without visible loss of quality since slight moiré was equally enhanced.

In tests of photography magazines cameras without aliasing filters are sometimes presented as if they had a "better" resolution and possibly even receive extra test-points in the evaluation. This is complete nonsense. In the images of artificial test charts, all visible structures are measured, including false artifacts, and not just the "true" structures from the real image. There is no way to differentiate between aliasing artifacts and real parts of the image in test charts. Cameras without aliasing filters are rated worse because the manufacturers do not push the sharpening to the top (for good reason). The negative image impression of moiré and color iridescence is not regarded in such "test reports" at all.

Digital cameras without aliasing filters are cameras with a built-in design flaw!

The mathematics behind the sampling theory is not particularly complicated. It is part of basic training of many engineering degree programs at colleges and universities.

In my various photo tours with the Fuji X-E1 (without aliasing filter) I have now noticed some images that suffer quite heavily under color streaks. An example is shown below. The forrest has a red simmer, which does not exist in reality.This image was developed with Capture One 7.0.2. There artifacts can be removed largely by turning on the internal Moiré Filter. At the same time, however, new image errors are introduced by the filter in other places. Therefore the filter has to be limited locally by an adjustment layer.The SilkyPix RAW converter does not show such artifacts. However, it generally produces images that have a lower resolution by some 10% compared to those from Capture One.

Some Internet forums are full of it: By omitting the alias filter, the image resolution can be increased significantly.Sounds logical: An APS-C camera without milky glass filters is a full-frame killer!

Well, I did the test. Three cameras with comparable lenses were used to thoroughly shoot test chart images, which were then analyzed with a MTF software. More on the testing conditions can be found here. The contestant were the Fuji X-E1 (without aliasing filter, but with the miraculous X-Trans sensor), the Canon EOS 600D (a hopeless APS-C camera with a conventional filter and conventional sensor) and the Canon EOS 5D Mark III (full frame, but also hopeless because of the alias filter).

Below are the objective results. The charts show the number of resolvable lines per picture height, with a contrast of 30%. Higher bars indicate better resolution. Three bars are shown per aperture, one for the image center (green), one for the edge regions (purple) and one for the outermost edge of the picture (orange).

The expert is surprised, amazed the layman: There is no (nil, zero, nothing) resolution advantage for the Fuji camera without aliasing filter. On the contrary, although the EOS 600D's nominal vertical resolution is only higher by 6% than the X-E1's, it is clearly visible in the results. The full frame 5D Mark III pulls both away.

Just to make this clear: I've done a lot more tests than shown above. Even with very good prime lenses instead of the zoom lenses. The results always have the same tendency.

And those who don't believe me can find a similar comparison with many more cameras in the well-recognized German photo magazine ColorFoto (issue 3/2013, pp. 30). The results are nonambiguous: No substantial resolution advantages for cameras without aliasing filters.

But why is that? Well, an alias filter works as a lowpass: Image details below the sensor resolution are transmitted to any significant extent. Only those details are blurred, which the sensor can not resolve anyhow. If the filter is properly designed and manufactured, you can see practically nothing of its presence. Only the moire is missing. I have compiled a practical explanation of the Nyquist-Limit in this report.

Sorry, the whole issue about substantial improvements of resolution by removing the alias-filter is nothing but a big hype.

P.s.: Many people confuse (perceived) sharpness with resolution (or detail). More on this can be found here. In short, a picture with high resolution (lots of details) may give an unsharp impression and a picture with much lower resolution may be perceived sharper. Resolution (detail) happens only at the time when the image is taken, sharpness can be increased in photoshop (with some restrictions) ...

Jonathan Moore Liles has written an excellent article on the usefulness (or lack thereof) of the Fuji X-Trans sensor.